It's a bit restrictive since the ability only triggers once a turn, but a powerful value engine in a deck that triggers it on your opponents turns
A commander can be cast from the command zone for its normal costs, plus an additional two mana for each previous time it's been cast from the command zone this game
Creatures with Celebration get all kinds of bonuses, such as power/toughness boosts and double strike, and Celebration is easy to enable with permanents that make Roles or creature tokens
Grist, the Hunger Tide is a three-mana Black and Green planeswalker that can do a bit of self-mill and make Insect tokens, sacrifice creatures to destroy creatures or planeswalkers, or drain the life from your enemies based on the number of creatures in your graveyard
If any abilities controlled by that opponent trigger but losing 3 life causes them to lose the game, none of those abilities will be put on the stack
If theyre actively doing these normal actions, youre growing River Song and damaging them